1. Field of the Invention
This invention relates to an improved form of apparatus for determining while drilling in the earth with a drill bit the positions of seismic wave reflecting geologic formations in the earth. More particularly, it relates to such an apparatus which utilizes signal processing and imaging techniques to characterize such geologic formations more precisely.
2. Description of the Prior Art
The use of the drill bit as a seismic source while drilling in the earth is taught by Widrow in U.S. Pat. No. 4,363,112, issued Dec. 7, 1982 and U.S. Pat. No. 4,365,322, issued Dec. 21, 1982. Using this type of downhole source, it is possible to detect a reflecting layer in the earth which is shallower than the depth of the drill bit. Upward going seismic energy radiating directly from the bit is shadowed at the surface by the target reflector and is detected by an array of surface geophones. It is also possible to detect a reflecting layer in the earth which is deeper than the depth of the drill bit. Direct and reflected waves arrive at the surface and the presence of the reflector is manifested in an interference pattern which can be observed by an array of surface geophones. Also, as shown in the above referenced Widrow patents, spatial sampling of the surface seismic field by geophone arrays is important to the imaging of reflective targets.